It is common to mount an LED die on a printed circuit board (PCB), or other substrate, for electrically connecting electrodes of the LED to conductive traces on the PCB. Then, a round reflector cup with a center hole is affixed to the PCB and surrounds the LED die. For phosphor conversion, the cup is then completely filled with a viscous phosphor mixture and cured to encapsulate the LED die. The combination of the LED die light and the phosphor light creates the desired overall light color, such as white light. The cup somewhat limits the side light emission of the LED die and redirects the side light in a generally forward direction.
In some cases, a hemispherical lens containing an encapsulant is affixed over the LED die to improve light extraction. This requires a large center hole in the cup to accommodate the lens.
One drawback of the above-described packaged LED is that the light emission profile of the phosphor light is very wide. Since the phosphor is at, or even slightly above, the rim of the conical cup, the phosphor light out of the cup is almost Lambertian. Since the LED die itself is fairly low in the cup, the direct light from the LED die is more sharply limited by the cup, so the direct light from the LED die exiting the cup is much narrower than Lambertian and much narrower than the phosphor light. Therefore, assuming the LED die emits blue light and the phosphor emits yellow light, there will be a yellow halo around the more central white light in the beam. This is often referred to as a phosphor halo effect.
Some examples of reflective cups filled with phosphor are shown in US publication 2013/0228810.
Encapsulation of an LED die is important to increase light extraction efficiency, and the encapsulant is designed to have an index of refraction (n) somewhere between the high index of the LED die (e.g., n=2.5-3 for a GaN LED) and air (n=1). In some LED dies, the LED die light exits from a top sapphire window with an index of about 1.8. The index of a conventional silicone encapsulant (including a lens) can be from 1.4 to 1.7. The encapsulation is thus designed to reduce the total internal reflection (TIR) inside the LED die. Encapsulation gain can account for a 10 to 20 percent increase in light output. The encapsulation shape is also designed to minimize the TIR at the encapsulant-air interface.
Dome-shaped encapsulation is popular since the rays of light emitted by the LED die impinge on the surface of the dome generally at right angles. This minimizes TIR. If an encapsulation shape resembles a rectangular prism, there will be relatively high TIR, due to the LED die light rays impinging on the flat encapsulant-air interface at low angles, and the symmetry of the shape does not allow light to escape. Therefore, encapsulants having a flat top surface (exposed to the air) are not used in actual products, although they may be illustrated in simplified schematic examples of packaged LEDs.
Some other known shapes of the encapsulant include pyramids, which have angles that break symmetry and allow the light to escape. However, TIR from the pyramid causes some of the light to be absorbed by the LED die and its mounting substrate. Some pyramid type structures have angular grooves cut in their outer surface to reduce TIR.
For various reasons, the user may not be content with a generally circular beam from a conical cup that has poor color uniformity due to the phosphor halo effect. Also, since lenses increase the height of the package, the user may want a shallower package that does not require a lens to encapsulate the LED die.
What is needed is a new design for a packaged LED that does not suffer from the drawbacks of the above-described prior art.